Material recycling has become an important industry in recent years due to decreasing landfill capacity, environmental concerns and dwindling natural resources. Many industries and communities have adopted voluntary and mandatory recycling programs for reusable materials. Solid waste and trash that is collected from homes, apartments and companies often combine several types of recyclable materials into one container. When brought to a processing center, the recyclable materials are frequently mixed together in a heterogeneous mass of material. Mixed recyclable materials include newspaper, clean mixed paper, magazines, aluminum cans, plastic bottles, glass bottles and other materials that may be recycled.
Disc screens are increasingly used to separate streams of mixed recyclable materials into respective streams or collections of similar materials. This process is referred to as classifying, and the results are called classification. A disc screen typically includes a frame in which a plurality of rotatable shafts is mounted in a parallel relationship. A plurality of discs is mounted on each shaft and a chain drive commonly rotates the shafts in the same direction. The discs on one shaft interleave with the discs on each adjacent shaft to form screen openings—i.e., the intended space—between the peripheral edges of the discs. The size of the intended space determines the dimension (and thus the type) of material that will fall through the screen. Rotation of the discs agitates the mixed recyclable materials to enhance classification. The rotating discs propel across the screen the larger articles which are too big to fall between the discs. The general flow direction extends from an input area where the stream of material pours onto the disc screen to an output where the larger articles pour off of the disc screen. The smaller articles fall between the discs onto another disc screen or a conveyor, or into a collection bin.
The prior art disc screens, however, have several shortcomings. First, the discs used to make up the screen are generally of the same diameter and therefore there is little lateral agitation, resulting in the majority of material remaining in the position where the material is initially deposited. The edges of the disc screen therefore are underutilized. Second, when the discs are of varying diameters, the change in diameter is by way of a step function. This then creates a gap between discs that is not intended to be used for sorting or classifying the material. Consequently, material can become “pinched” in these unintended gaps, damaging the discs and reducing the overall efficiency of the disc screen.
What is therefore needed is a disc for use in a disc screen that overcomes these deficiencies.